The purpose of this article is to follow on from our explanation of the 555 timer IC by demonstrating some simple yet interesting, noisy and plain annoying uses of the 555. They are by no means that complex, and intended to help move theory into practice.

Button de-bouncer

De-bouncer? How does one bounce a button in the first place? Many years ago I bounced a button on the arcade Sonic the Hedgehog – hit it so hard it popped out and bounced over the table… But seriously, when working with digital logic circuits, you may need to use a momentary button to accept user input. For example, to pulse a trigger or so on. However with some buttons, they are not all that they seem to be. You press them once, but they can register multiple contacts – i.e. register two or more ‘presses’ for what seems like only one press. This could possibly cause trouble, so we can use a 555 timer monostable circuit to solver the problem. In our de-bounce example, when the button is pressed, the output is kept at high for around half a second. Here is the schematic:

What we have is a basic monostable timer circuit. For my example the output delay (t) is to be half a second. The formula for t is: t=1.1xR1xC1. The closest resistor I had at hand was 2k ohms, so to find the required value for C1, the formula is rearranged into: C1=t/(1.1xR1). Substituting the values for t and R1 gives a value of C1 as 227.274 uF. So for C1 we have used a 220 uF capacitor.

Now for a visual demonstration of the de-bouncer at work. In the following video clip, the oscilloscope is displaying the button level on the lower channel, and the output level on the upper channel. The button level when open is high, as the 555 requires a low pulse to activate. The output level is normally low. You can see when the button is pressed that the button level momentarily drops to low, and then the output level goes high for around half a second:

Make some noise

As we know the 555 can oscillate at frequencies from less than 1Hz to around 500 kHz. The human ear can theoretically hear sounds between (approximately) 20 and 20 kHz. So if we create an astable timing circuit with an output frequency that falls within the range of the human ear, and connect that output to a small speaker – a range of tones can be emitted.

The circuit required is a standard 555 astable, with the output signal heading through a small 8 ohm 0.25 watt speaker and a 4.7 uF electrolytic capacitor to ground. The capacitor stops any DC current flowing to ground, without this we will overload the current-handling ability of the 555. (I couldn’t help myself by trying it without the capacitor – pulled 550 mA from the 555 before it stopped working…). To choose the values of R1 and C1 to emit out required frequency, the following formula is used: f (frequency) = 1.4 / {(R1 + [2 x R2]) x C1}. To cover the range required, a 100k ohm trimpot was used for R1. Here is the resulting schematic:

The input voltage can fall within the specification of the 555, however for optimum results a supply of between 5 and 9 volts DC should be used. In the following demonstration, we used a 9V supply. The purpose of the video is to learn the relationship between the tones and their frequencies. You can see the frequency on my old counter and hopefully hear the result:

Our next example is to create a siren effect, using two 555 circuits – one for a low frequency and one for a high frequency. To determine the value for R1 for the low and high frequency, I used the previous circuit and chose two tones that were quite different, and measured the resistance of the trimpot (R1) at those frequencies. My R1 value for the ‘low’ tone is 82k ohm and 36k ohm for the ‘high’ frequency.

The switching between low and high frequency will be handled by a 4047 multivibrator – the Q and Q outputs will control NPN transistors. The transistors are used as switches to allow current to flow from the supply to the 555 high or low tone circuit. We use this method as the 4047 is not able to source enough current to drive the 555 circuits. Here is the schematic:

Don’t forget to connect pin 14 of the 4047 to supply voltage. This circuit has been tested with a supply voltage between 5 and 12 volts. As the supply voltage increases, so does the amplitude of the square wave emanating from the 555 output pins, which in turn in creases the volume of the siren. At 5 volts, the entire circuit drew only 20 milliamps. Speaking of which, you can listen to a recording of the output here. If you wish to alter the time for each tone, adjust the value of what is the 47k ohm resistor on pins 2 and 3 of the 4047.

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My reason for doing this is to demonstrate another way to create a square-wave output for digital circuits (astable mode) and also generate single pulses (monostable mode). Sometimes one can get carried away with using a microcontroller by default – and forget that there often can be simpler and much cheaper ways of doing things. And finally, the two can often work together to solve a problem.

What is a multivibrator? In electronics terms this means more than one vibrator. It creates an electrical signal that changes state on a regular basis (astable) or on demand (monostable). You may recall creating monostable and astable timers using the 555 timer described in an earlier article. One of the benefits of the 4047 is being able to do so as well, but with fewer external components. Here is the pinout diagram for a 4047 (from the Fairchild data sheet):

Note that there are three outputs, Q, Q and OSC out. Q is the normal output, Q is the inverse of Q – that is if Q is high, Q is low – at the same frequency. OSC output provides a signal that is very close to twice the frequency of Q. We will consider the other pins as we go along. In the following small video, we have LEDs connected to all three outputs – you can see how Q and Q alternate, and the increased frequency of OSC out:

That was an example of the astable mode. The circuit used is shown below. The only drawback of using a 4047 is that you cannot alter the duty cycle of your astable output – it will always be 50% high and 50% low. The oscillator output is not guaranteed to have a 50% duty cycle, but comes close. The time period (and therefore the frequency) is determined by two components – R1 and the capacitor:

The values for R2~R4 are 560 ohms, for the LEDs. R1 and the capacitor form an RC circuit, which controls the oscillation frequency. How can we calculate the frequency? The data sheet tells us that time (period of time the oscillator is ‘high’) is equal to 4.4 multiplied by the value of R1 and the capacitor. As the duty cycle is always 50%, we double this value, then divide the result into one. In other words:

And as the frequency from the OSC out pin is twice that of Q or Q, the formula for the OSC out frequency is:

However the most useful formula would allow you to work with the values of R and C to use for a desired frequency f:

When calculating your values, remember that you need to work with whole units, such as Farads and Ohms- not microfarads, mega-ohms, etc. This chart of SI prefixes may be useful for conversions.

The only thing to take note of is the tolerance of your resistor and capacitor. If you require a certain, exact frequency try to use some low-tolerance capacitors, or replace the resistor with a trimpot of a value just over your required resistor value. Then you can make adjustments and measure the result with a frequency counter. For example, when using a value of 0.1uF for C and 15 k ohm for R, the theoretical frequency is 151.51 Hz; however in practice this resulted with a frequency of 144.78 Hz.

Don’t forget that the duty cycle is not guaranteed to be 50% from the OSC out pin. This is shown in the following demonstration video. We measure the frequency from all three output pins, then measure the duty cycle from the same pins:

(The auto-ranging on that multimeter is somewhat annoying).

Now for some more more explanation about the 4047. You can activate the oscillations in two ways, via a high signal into pin 5 (pin 4 must then be low) or via a low signal into pin 4 (and pin 5 must be low). Setting pin 9 high will reset the oscillator, so Q is low and Q is high.

The monostable mode is also simple to create and activate. I have not made a video clip of monstable operation, as this would only comprise of staring at an LED. However, here is an example circuit with two buttons added, one to trigger the pulse (or start it), and another to reset the timer (cancel any pulse and start again):

The following formula is used to calculate the duration of the pulse time:

Where time is in seconds, R is Ohms, and C is Farads. Once again, the OSC output pin also has a modified output – it’s time period will be 1.2RC.

To conclude, the 4047 offers a simple and cheap way to generate a 50% duty cycle square wave or use as a monostable timer. The cost is low and the part is easy to source. As always, avoid the risk of counterfeit ICs and get yours from a reputable distributor. Living in Australia, mine came from element-14. Thanks to Fairchild Semiconductor for product information from their 4047 data sheet.

Have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other – and we can all learn something.